As far as protection is concerned, I think a thermal switch mounted to the waterblock, and wired to power down the computer in the event of overheating is the safest and simplest way to go.
The fundamental purpose of the protection is to protect the processor from overheating.
Pump fails - thermal switch saves the CPU.
Rad gets totally clogged with dust - thermal switch saves the CPU.
Apartment next to yours is on fire and ambient gets to 80C - thermal switch saves the CPU. (At least temporarily.)
Thermal switches are available in a large range of trip temperatures at Digi-Key.